Various organic and inorganic materials have been proposed for incorporation into coatings in order to impart antimicrobial properties. Inorganic, “naturally occurring”, antimicrobial agents are often desired as a “green” alternative to synthetically produced organic antimicrobial agents. These materials often employ metals, especially silver, zinc, gold, and/or copper, which are believed to have relatively low environmental and toxicological effects and high antimicrobial activity. Antimicrobial agents that incorporate ionic forms of these metals, often through an ion-exchange type mechanism, are sometimes used. Exemplary ion-exchange type antimicrobial agents include those wherein the ion-exchange carrier particles are ceramic particles, such as zeolites.
A problem associated with inorganic ion-exchange type antimicrobial agents, however, is their tendency to cause discoloration of the composition into which they are incorporated, particularly when silver ions are used. This discoloration may arise from, it is believed, an interaction of silver ions on the surface of the carrier particles and/or exchanging out of the carrier particles with other compounds or ions present in the composition into which the antimicrobial agent is incorporated. Such discoloration can be particularly problematic in coatings applications where decorative properties are often critical.
As a result, efforts have been made to reduce the discoloration of coatings that use inorganic ion-exchange type antimicrobial agents. For example, partial replacement of the ion-exchangeable ions in a zeolite with ammonium has been taught to reduce discoloration. However, as described in U.S. Pat. No. 4,938,955 (“the '955 patent”) and U.S. Pat. No. 6,436,422 (“the '422 patent”), it is often necessary to further add certain organic discoloration inhibitors in addition to the ammonium ion containing zeolite in order to adequately prevent discoloration of the resin in which the zeolite is incorporated. As described in the '422 patent, however, the addition of the organic discoloration inhibitors described in the '955 patent can be problematic.
It would be desirable to provide antimicrobial compositions that employ ion-exchange type inorganic antimicrobial agents, but that do not exhibit the degree of discoloration historically associated with coatings incorporating such antimicrobial agents and which do not require the use of the organic the discoloration inhibitors described in the '955 patent to achieve such discoloration inhibition, while not significantly impacting the antimicrobial efficacy of the ion-exchange type inorganic agents.